This invention relates to desmodromic valve systems, and more particularly to desmodromic valve systems which provide direct bidirectional displacement of a valve stem without the aid of a rocker arm.
A desmodromic valve system positively opens and closes a valve in an internal combustion engine. This is in contrast to the conventional system in which the valve is positively opened with a cam but closed with a return spring.
The main benefit of a desmodromic system is the prevention of valve float. In traditional spring valve actuation, as engine speed increases, the inertia of the valve tends to overcome the spring's ability to close the valve completely before the piston reaches TDC (Top Dead Center). In severe cases, the piston contacts the open valve and causes damage to both engine parts. More generally, if a valve does not completely return to its seat before combustion begins, it can allow combustion gases to escape prematurely, leading to a reduction in cylinder pressure which causes a major decrease in engine performance. This can also overheat the valve, possibly warping it and leading to catastrophic failure. The traditional remedy for valve float is to use a stiffer return spring. This increases the seat pressure of the valve, i.e., the static pressure that holds the valve closed, and reduces valve float at higher engine speeds. However, the engine has to work harder to open the valve. The higher forces between spring and cam cause higher stress on the parts resulting in higher temperature and faster wear or failure in the valve drive system. A desmodromic system can avoid the problem to some extent because, although it has to work against the inertia of the valve opening and closing, it does not have to overcome the energy of the spring.
Despite their advantages, desmodromic valve drive systems have had limited success in commercial application for various reasons such as design complexity, poor reliability, and valve train binding. Numerous approaches to the various problems have been taken since the earliest days of engine development, more than a hundred years ago, as evidenced by the following patents:
Pat. No.Inventor(s)Issue Date1,644,059HolleOct. 24, 19271,937,152JünkNov. 28, 19333,183,901ThuesenMay 18, 19653,430,614MeachamMar. 4, 19694,711,202BakerDec. 8, 19874,763,615FrostAug. 16, 19884,887,565BothwellDec. 19, 19895,048,474Matayoshi et al.Sep. 17, 19915,058,540MatsumotoOct. 22, 19916,276,324Adams et al.Aug. 21, 20016,487,997PalumboDec. 3, 20026,948,468DecuirSep. 27, 20056,951,148BattloggOct. 4, 2005
However, presently, all known desmodromic valve designs have drawbacks which make them undesirable for use in several significant applications, such as production automobiles, and there is no obvious path to a better solution.